1. Field of the Invention
The present invention relates to a data writing method for a magnetoresistive effect element, and a magnetic memory.
2. Description of the Related Art
Various types of solid-state magnetic memories have been suggested heretofore. In recent years, a magnetoresistive effect memory using a magnetoresistive effect element which demonstrates a giant magnetoresistive (GMR) effect has been suggested. Among others, a magnetoresistive random access memory (MRAM) using ferromagnetic tunnel junction which demonstrates a tunneling magnetoresistive (TMR) effect attracts attention in particular.
A basic structure of an MTJ (magnetic tunnel junction) element of the ferromagnetic tunnel junction is a three-layer structure including a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer. Electrons tunnel into the insulating layer due to a magnetization, and a current flows through the magnetoresistive effect element. A junction resistance value of the MTJ element varies in accordance with a cosine of a relative angle of the magnetizations of the first and second ferromagnetic layers. Therefore, the junction resistance value becomes a minimal value when the magnetizations of the first and second ferromagnetic layers are parallel, and it becomes a maximal value when they are anti-parallel. This is called a TMR effect. A change in resistance value due to the TMR effect may exceed 300% at room temperature in some cases.
When the MTJ element is used for magnetoresistive effect memory, at least one ferromagnetic layer is regarded as a magnetization invariable layer (which is also referred to as a base layer, a reference layer, or a fixed layer) to fix a magnetizing direction thereof, and the other ferromagnetic layer is determined as a magnetization variable layer (which is also referred to as a free layer, or a recording layer). In such an MTJ element, “0” or “1” as binary information is associated with a state that magnetizing directions of the magnetization invariable layer and the magnetization variable layer are parallel or a state that they are anti-parallel, information (data) is stored in the MTJ element.
As one of the data writing schemes, there is a scheme of flowing a current through a write interconnect line provided separately from a memory cell and reversing the magnetization of the magnetization variable layer based on a magnetic field generated from this current. Such a writing scheme is called a current magnetic field writing scheme. As a writing scheme different from the current magnetic field writing scheme, there is a spin transfer magnetization reversal scheme. According to the spin transfer magnetization reversal scheme, directly flowing a current through an element enables reversing the magnetization of the magnetization variable layer based on a spin torque injected from the magnetization invariable layer.
Data is read by flowing a current through the ferromagnetic tunnel junction and detecting a difference in resistances of an element caused due to the TMR effect.
A memory cell array in a MRAM is formed by arranging many memory cells each including an MTJ element. As a configuration of the memory cell, a select transistor is provided in each memory cell like, e.g., a DRAM so that an arbitrary MTJ element can be selected. Further, a peripheral circuit is incorporated in the same chip as the memory cell array.
To realize a memory having a large storage capacity, the element must be miniaturized, and a cell share ratio in the chip must be increased. The spin transfer reversal scheme is characterized in that the element can be miniaturized as compared with the current magnetic field writing scheme, and it is anticipated as a writing scheme suitable for realizing a MRAM having a large storage capacity.
In a MRAM using the spin transfer magnetization reversal scheme, element characteristics of the MTJ element fluctuate (see, e.g., H. Tomita et al., Appl. Phys. Express 1 (2008) 061303). When an excessive write current equal to or above a threshold reversing value is supplied to the MTJ element, there occurs a problem of an erroneous writing phenomenon that data written in the MTJ element is lost before the write current falls, and unintended data is thereby written.